The planetary boundary layer (PBL), the lowest part of the troposphere, contains the majority of the atmospheric water vapor. The rapidly changing spatial and temporal distribution of water vapor contained within this part of the atmosphere influences convective processes that drive weather and circulation patterns and affects radiative transfer, the water cycle, and soil moisture. The ability to continuously measure water vapor distributions within the lower troposphere has been identified as a high priority measurement capability needed by both the weather forecasting and climate science communities. Atmospheric studies have listed high-resolution vertical profiles of humidity in the lower troposphere and PBL height as high priority observations that need to be addressed for the next generation mesoscale weather observation network. In addition, these observations are of high importance to the National Weather Service and other Federal agencies for evaluation of forecast impact in severe weather plus quantitative precipitation forecasts. Yet, accurate, high-resolution, continuous measurements of water vapor remain a key observational gap for the mesoscale weather and climate process studies communities.
Local soundings of high vertical resolution atmospheric state parameters via radiosonde measurements combined with global coverage of low vertical resolution state parameters via satellite-based measurements form the backbone of observations used for weather forecasting. But the limited spatial and temporal resolution of the current state of technology prohibits observations of key atmospheric features required for accurate forecasting of mesoscale high-impact weather events like thunderstorms. Passive remote sensors such as infrared and microwave radiometers are useful at low ranges close to the surface but in general provide low vertical resolutions. The atmospheric emitted radiance interferometer (AERI) is a passive remote sensing instrument that utilizes an interference technique to retrieve atmospheric emitted radiance. Starting with an initial temperature and water vapor profiles based on statistical models, an iterative solution to the radiative transfer equations is utilized to reproduce the measured atmospheric emitted radiance. This iterative solution provides the final temperature and water vapor profiles for clear sky conditions up to approximately 3 km with a 250 m range resolution. During cloudy conditions, retrieval of temperature and water vapor profiles are sensitive to the cloud properties leading to larger errors in the retrieved temperature and water vapor profiles. During cloudy conditions a separate measurement of the cloud base height is often required.
Raman LIDAR is an active remote sensing technique that is capable of monitoring water vapor throughout the troposphere. The Raman LIDAR technique utilizes a high power pulsed laser transmitter to actively illuminate the atmosphere. Range information is determined by the time of flight of the laser pulse traveling from the beam transmitter to the scatterer (aerosols, clouds, and molecules) and back to the receiver. The frequency shift due to the inelastic Raman scattering allows the scattering molecule to be identified while the intensity of the scattered signal can be used to determine the water vapor mixing ratio. Raman LIDAR typically require a high pulse energy and large receiver aperture due to the small scattering cross section associated with inelastic Raman scattering. Furthermore, Raman LIDAR typically require a calibration technique based on an ancillary measurement for quantitative water vapor retrievals.
Differential absorption LIDAR (DIAL) is another active remote sensing technique used to directly measure column and range resolved profiles of atmospheric trace gases. A laser source emits a pulse of light (typically a few nanoseconds to 1 microsecond in duration), and as the pulse propagates, the photons interact with particles in the atmosphere. Some of these interactions, such as Mie and Rayleigh scattering, result in backscattered photons. The photons are collected by a detector and recorded as function of time. This time-of-flight data has a direct correspondence with the range (distance) at which the scattering event occurred. DIAL utilizes a laser transmitter capable of operating at two closely spaced wavelengths, one wavelength which is located at or near the absorption feature for the molecule of interest, referred to as the online wavelength and the other located away from the same absorption feature, referred to as the offline wavelength. If the online and offline wavelengths are closely spaced, then the only difference between the return signals results from molecular absorption. Using an a priori knowledge of the differential absorption cross section of the molecule of interest available, for example, in the HIgh-resolution TRANsmission molecular absorption database (HITRAN), the ratio of the online and offline return signals over a selected range within the atmosphere can be used to determine a molecular number density. The DIAL technique provides the advantages of not requiring a calibration or ancillary measurements, and providing a direct measurement of the water vapor number density. However, the DIAL technique requires a pulsed laser with high spectral fidelity and agility capable of operating at two separate wavelengths.
Co-axial or concentric geometry LIDARs are used in the LIDAR community as a way to propagate the transmitted beam into the receiver field of view. However, these systems often use a separate mirror concentric with the telescope secondary mirror for laser transmission. Greater stability and a larger beam expansion, required for eye safety in compliance with Federal Aviation Administration regulations, can be achieved with a shared telescope design approach which utilizes the receiver telescope to expand and transmit the outgoing laser beam. Prior shared telescope designs have illuminated the entire primary mirror to expand the beam with the telescope, however, losing light blocked by the secondary mirror. For a micropulse LIDAR system using a low-power laser transmitter, inefficiently transmitting the beam through the telescope impacts performance. Higher efficiency designs have been achieved by shaping the transmit beam into an annulus with an axicon lens pair. Beam shaping alone cannot solve the performance problems experienced by prior micropulse LIDAR systems if the beam is expanded to the full diameter of the telescope, however, because prior designs have needed polarization transmit/receive switches to separate transmit and receive paths in the shared telescope. The polarization transmit/receive switches have provided for poor isolation between transmit and receive paths.
Removing solar background light during the daytime and when there are cloudy conditions from the return signal of a LIDAR is also important. Prior instruments used a narrowband filter having a 250 pm full width half maximum (FWHM) bandpass, permitting measurements up to 3 km in altitude to be obtained. Greater altitudes, and therefore better background light filtering are needed, however.
Prior DIAL designs have used a fiber coupled MEMS switch to alternate between the online and offline wavelength which, exhibiting approximately 1 ms 10/90 switching time. The data acquisition system utilized a single channel of a four channel multi-channel scalar photon counting card which had 2 buffers to allow for continuous read and write operations to occur. To avoid mixing the signals within the data acquisition system, a first laser frequency was transmitted for 3 seconds, followed a 3 second dead time when the wavelength switch was changed, then a second laser frequency was transmitted for 3 seconds. Switching with the MEMS switch created several performance limitations. First, the dead time resulted in a 60% duty cycle (i.e., the reported water vapor integration time of 20 minutes was equivalent to a 33 minutes temporal resolution). Second, wavelength switching on timescales of several seconds can result in errors due to rapid changes between the online and offline backscattered signals resulting from fluctuations in the aerosol distributions on short time scales. A higher duty cycle DIAL instrument would allow the acquisition of more robust data collection.
What is needed is a micropulse differential absorption LIDAR instrument that is capable of autonomous long term field operation under an expanded set of atmospheric conditions, such as during the daytime, with cloud cover, and during times of rapidly changing atmospheric conditions. Ideally, the micropulse differential absorption LIDAR device will be eye-safe, will be mechanically and thermally stable for the beam transmitter and receiver, will have an improved duty cycle, will provide excellent isolation between the transmitted and received signals, and will offer improved performance.